In an internal combustion engine with controlled ignition, the term misfire or misfiring indicates a phenomenon of incorrect combustion within one or more cylinders; the misfire is in particular known as an ignition misfire when it is due to the inadequate or lacking energy of the spark generated by the spark plug, and is known as an injection misfire when it is due to a fuel supply which is inadequate or lacking.
The presence of a misfire is particularly damaging since incorrect combustion is detrimental to engine performance, increases the level of pollutant emissions from the engine and may cause permanent damage to the catalyst. For this reason, European standards on automobile vehicle emission limits require misfiring to be detected efficiently and for drivers to be informed of the presence of a misfire by means of an indicator light disposed on the dashboard. In particular, it is necessary, under European regulations on automobile vehicle emission limits, to indicate an increase in pollutant emission levels when the number of misfires in a first interval (for instance 1000 TDC—top dead centres) exceeds a first threshold, and to indicate permanent deterioration of the catalyst when the number of misfires in a second interval (for instance 200 TDC—top dead centres) exceeds a second threshold.
At present, misfiring is detected indirectly, i.e. by analysing the instantaneous value of the angular acceleration of the drive shaft or the instantaneous value of the drive torque, as a direct analysis of combustion by means of sensors disposed inside each cylinder is not practicable for reasons of cost.
The most widespread method for detecting misfiring involves analysing the angular acceleration of the drive shaft; in particular, the signal supplied by the phonic wheel is used to calculate the value of the angular acceleration of the drive shaft at predetermined angular positions of this drive shaft, and misfiring is detected if the absolute value of the angular acceleration of the drive shaft is higher than a predetermined threshold value.
It has been observed, however, that this method is not very reliable as the absolute value of the angular acceleration of the drive shaft may also exceed the threshold value as a result of causes unconnected with misfiring, such as, for instance, the coupling or uncoupling of the compressor of the air conditioning plant, the roughness of the road, or even abrupt decelerations. Moreover, in the case of a single misfire, the value of the angular acceleration of the drive shaft may be supplemented by torsional oscillations which cause the absolute value of the angular acceleration of the drive shaft to exceed the threshold value at several successive instants; in this case, a single misfire is erroneously detected as a plurality of successive misfires.
A first solution to the problems discussed above is disclosed in European Patent Application EP-0637738-A1 which discloses a method for detecting misfiring in an internal combustion engine by analysing the angular acceleration of the drive shaft; in order to try to eliminate the influence of disturbances, the value of the angular acceleration of the drive shaft is processed by creating a so-called cyclicity index which, compared with appropriate thresholds, indicates the presence of misfiring.
However, the method for detecting misfiring as disclosed in European Patent Application EP-0637738-A1 also tends erroneously to detect a plurality of successive misfires rather than a single misfire as a result of the oscillations of the value of the angular acceleration of the drive shaft triggered by a single misfire.